Defense readiness conditions vary between many commands and have changed over time,[2] and the United States Department of Defense uses exercise terms when referring to the DEFCONs.[6] This is to preclude the possibility of confusing exercise commands with actual operational commands.[6] On 12 January 1966, NORAD "proposed the adoption of the readiness conditions of the JCS system", and information about the levels was declassified in 2006:[7]

After NORAD was created, the command used different readiness levels (Normal, Increased, Maximum) subdivided into eight conditions, e.g., the "Maximum Readiness" level had two conditions "Air Defense Readiness" and "Air Defense Emergency".[7] In October 1959, the JCS Chairman informed NORAD "that Canada and the U.S. had signed an agreement on increasing the operational readiness of NORAD forces during periods of international tension."[7] After the agreement became effective on 2 October 1959,[7] the JCS defined a system with DEFCONs in November 1959 for the military commands.[2] The initial DEFCON system had "Alpha" and "Bravo" conditions (under DEFCON 3) and Charlie/Delta under DEFCON 4, plus an "Emergency" level higher than DEFCON 1 with two conditions: "Defense Emergency" and the highest, "Air Defense Emergency" ("Hot Box" and "Big Noise" for exercises).[7]

According to documents declassified in 2016, the move to DEFCON 3 was motivated by Central Intelligence Agency reports indicating that the Soviet Union had sent a ship to Egypt carrying nuclear weapons along with two other amphibious vessels.[10] Soviet troops never landed, though the ship supposedly transporting nuclear weapons did arrive in Egypt. Further details are unavailable and may remain classified.

Over the following days, the various forces reverted to normal status with the Sixth Fleet standing down on November 17.[11]

The television series Deutschland 83 depicts the events which occurred in 1983 when the level was raised to a simulated DEFCON 1 during NATO exerciseAble Archer 83. In the story line, briefing papers released under the Freedom of Information Act reveal that Operation Able Archer, a major war games exercise conducted in November 1983 by the US and its NATO allies, was so realistic it made the Soviets believe that a nuclear strike on its territory was a real possibility.[14]